offers an attractive prospect for using this material in future two-dimensional electronic devices. Here we demonstrate for the first time fully encapsulated ultrathin (down to bilayer) black phosphorus field effect transistors in Van der Waals heterostructures to preclude their stability and degradation problems which have limited their potential for applications. Introducing monolayer graphene in our device architecture for one-atom-thick conformal source-drain electrodes enables a chemically inert boron nitride dielectric to tightly seal the black phosphorus surface. This architecture, generally applicable for other sensitive two-dimensional crystals, results in stable transport characteristics which are hysteresis free and identical both under high vacuum and ambient conditions. Remarkably, our graphene electrodes lead to contacts not dominated by thermionic emission, solving the issue of Schottky barrier limited transport in the technologically relevant two-terminal field effect transistor geometry.